1. Field of the Invention
The present invention relates to an illumination optical system, an illumination unit, and an image projection apparatus employing the same. More particularly, the present invention relates to an illumination optical system having an improved structure in which light may be effectively condensed even when a light source cannot be regarded as a point light source, an illumination unit, and an image projection apparatus employing the same.
2. Description of the Related Art
Conventionally, various illumination optical systems that condense light emitted from a light source and illuminate a proper region with a desired light distribution have been proposed. Recently, solid light-emitting devices, such as light emitting diodes (LED's), have been widely used as light sources. Additionally, such illumination optical systems have been used as a portion of image projection apparatuses, such as projectors, rear projection televisions, and light-transmitting apparatuses.
For example, a light-emitting apparatus is disclosed in Japanese Patent Laid-open Publication No. hei 10-311944 (hereinafter, referred to as patent literature 1). In FIG. 1, 5 or 9, the light-emitting apparatus includes a reflector whose cross-section is an oval, partial circular arc-shaped reflective surface, a lamp that emits light onto a focus of the reflective surface, and a concave lens disposed between two focuses of the reflector.
Additionally, Japanese Patent Laid-open Publication No. 2003-35883 (hereinafter, referred to as patent literature 2) discloses an optical output device. In FIGS. 5 and 7, the optical output device includes a point light source, a reflector that forms a homed reflective surface on its rear surface, a first lens group having a positive refractive power and disposed on front surfaces of the point light source and the reflector, respectively, and a second lens group disposed in the vicinity of an optical axis of the first lens group, wherein the second lens group has a refractive power that is negative or 0 in the vicinity of the optical axis and its circumferential refractive power is positive.
Additionally, Japanese Patent Laid-open Publication No. 2005-208571 (hereinafter, referred to as patent literature 3) discloses an illumination apparatus and a projector. In FIG. 1, 8 or 10, the illumination apparatus includes a light emitting diode (LED) element and a collimator optical system, wherein the collimator optical system includes a meniscus lens whose light source surface is a concave surface, and an aspherical lens having an aspherical surface, sequentially from the LED element. A condenser optical system having a decreasing refractive power farther from the optical axis, is disposed on a rear end of the optical system. Light emitted from a circumference of the light source can be approximately condensed in the range of condensing light emitted from the central part of the light source.
An illumination optical system using a solid light-emitting element, such as an LED, an illumination unit, and an image projection apparatus using the same needs to be made small and to have high brightness. To implement high brightness, the light-emitting area of a light source, such as an LED, is increased or a plurality of light sources are disposed so that the optical output of a light source can be increased. In the illumination optical system, preferably, light emitted from the light source can be guided with high efficiency in a direction toward a predetermined angle.
However, the above-described illumination optical system, the illumination unit using the same, and the image projection apparatus have the following problems.
In the technique disclosed in the patent literature 1, the light source is disposed in a focus position of the oval-shaped reflector so that its position can be adjusted. Thus, a desired light distribution can be obtained when a lamp is a point light source but if the size of the lamp is relatively increased compared to the lens or the reflector, the entire light emitted from all light-emitting surfaces of a so-called surface light source cannot be guided to a direction toward a predetermined angle, and because good light distribution cannot be consequently obtained, optical efficiency is deteriorated.
The technique disclosed in the patent literature 2 has a different construction from that of the patent literature 1 but there is a common point that a light distribution is adjusted based on a point light source. Thus, when the size of the light source is relatively large, good light distribution cannot be obtained and optical efficiency is deteriorated.
Because the invention disclosed in the patent literature 3 uses a lens optical system as a collimator optical system, even though light emitted from the circumference of the light can be condensed, the light is defined as the range of a numerical aperture (NA) of the lens optical system. Thus, to improve optical efficiency, a collimator optical system having a large NA needs to be constructed. However, because the construction of a lens is complicated or the diameter of an effective lens is increased, the illumination optical system having a small size and good optical efficiency, the illumination unit, and the image projection apparatus employing the same cannot be implemented.
Accordingly, a need exists for an illumination optical system in which light may be effectively condensed regardless of the light source.